Add weakening rule

used by Carcara.

Addresses #45

spec/changelog.tex

@@ -11,6 +11,8 @@ Proof rules:
   of equality explicitly but as an equivalence. Note that in principle this
   could be done with the rule \proofRule{symm} above, but would require a long
   and tedious use of \proofRule{subproof} for each direction of the equivalence.
+  \item Addition of the rule \proofRule{weakening} to express weakening of a
+        clause.
 \end{itemize}
 
 \noindent